JP4954981B2 - A high yield ratio cold-rolled steel sheet excellent in formability and its manufacturing method. - Google Patents

Description

  The present invention relates to a composite-added IF (Interstitial Free) cold rolled steel sheet of Nb and Ti used for materials such as automobiles and home appliances. More specifically, the present invention relates to a high formability IF cold-rolled steel sheet whose yield strength is enhanced by the distribution of fine precipitates and a method for producing the same.

  Cold rolled steel sheets used for automobiles and home appliances are required to have normal temperature aging resistance and bake hardenability as well as ensuring strength and formability.

  Aging is a kind of deformation aging phenomenon that occurs while hardening occurs due to solid solution elements (C, N) adhering to dislocations. Since aging induces a defect called Stretcher Strain, it is important to ensure normal temperature aging resistance.

  In the bake hardening characteristic, a small amount of carbon remains in a solid solution state, and after press molding, the strength is increased by the solid solution carbon during a coating and drying process. When the steel sheet has bake-hardening characteristics, the difficulty of press formability due to the increase in strength can be solved.

  When aluminum killed steel is subjected to batch (box) annealing, it is possible to give room temperature aging resistance and bake hardening characteristics. However, since batch annealing has a long annealing time, there is a disadvantage that the productivity is low and the material deviation is severe for each part. Further, the bake hardening amount (difference between the yield strength before the coating treatment and the yield strength after the coating treatment) is about 10-20 MPa, and the increase in the yield strength is low.

  Therefore, an IF steel (Interstitial Free Steel) has been developed that can provide normal temperature aging characteristics and bake hardening characteristics by continuous annealing while adding charcoal and nitride forming elements such as Ti and Nb.

  In Patent Document 1, 0.4 to 0.8% Mn and 0.04 to 0.12% P are added to Ti-based IF steel to ensure strength. In an extremely low carbon component IF steel, P causes a problem of secondary work embrittlement due to grain boundary segregation.

  In Patent Document 2, Mn, a solid solution strengthening element, is added in excess of 0.9 to 3.0% instead of P to ensure high strength.

  In Patent Document 3, 0.5 to 2.0% of Mn, Al, B and the like are added in place of P to improve secondary work brittleness resistance as well as strength and workability.

  In Patent Document 4, high strength is secured by using both solid solution strengthening elements of Mn and Si while lowering P. In this prior art, Mn is used up to 0.5%. In the case of Al, 0.1% is added as a deoxidizer, and in the case of N, it is controlled to 0.01% or less as an impurity. When the content of Mn is increased, the plating characteristics are not good.

  In Patent Document 5, 0.5 to 2.5% of Cu is added to IF steel to form a precipitated phase of ε-Cu to ensure strength. High strength is ensured by the precipitation phase of ε-Cu, but the processing characteristics are not good.

  In Patent Document 6 and Patent Document 7, a technique for improving workability or surface defects by using carbide as a precipitation nucleus of carbide is proposed. In Patent Document 6, CuS is precipitated during the rough rolling process of IF steel to which Cu is added in an amount of 0.005-0.1%, and Cu-Ti-C-S precipitates are formed using CuS as a nucleus in the hot rolling process. Secured. It is claimed that nuclei forming a plane {111} parallel to the plate surface increase around the Cu—Ti—C—S precipitate in the recrystallization process, thereby improving workability. In the above-mentioned Patent Document 7, 0.01 to 0.05% of Cu is added to IF steel and CuS precipitates are used as carbide precipitation nuclei to reduce solid solution C and improve surface defects.

  In these prior arts, carbides are present in the final product (cold-rolled steel sheet) by using coarse CuS precipitates during the manufacturing process. Further, sulfide-forming elements such as Ti and Zr are added in an amount of S or more in terms of the element quantity ratio, and most of S reacts with Ti or Zr from Cu.

  On the other hand, in Patent Document 8 and Patent Document 9, Cu and P are added in combination in order to ensure corrosion resistance in the bake hardened IF steel. These prior arts are steels in which Cu is added in an amount of 0.05 to 1.0% in order to ensure corrosion resistance, but Cu is actually added in an excess amount of 0.2% or more.

  In Patent Document 10 and Patent Document 11, BH content (difference in yield strength before and after baking) 30 MPa is obtained by dissolving carbon sulfide (Ti—C—S system) in carbide during solidification and resolving at the grain boundary during reheating annealing. Techniques for ensuring the above have been proposed.

  In the above prior art, the strength is increased by solid solution strengthening or the strength is ensured by using a precipitated phase of ε-Cu. In the case of Cu, in addition to the precipitation phase of ε-Cu, it is used as a corrosion resistant side surface or carbide precipitation nucleus. These prior arts do not lower the in-plane anisotropy index together with the high yield ratio (yield strength / tensile strength).

  If the yield strength of IF steel is higher than that of the same tensile strength, that is, if the yield ratio is high, the thickness of the steel sheet can be reduced, and the weight can be reduced. In addition, when the in-plane anisotropy index is low, the generation of wrinkles is reduced during processing, and there is an advantage that the generation of ears is small after processing.

Japanese Published Patent Publication No. 57-041349 Japanese Published Patent Publication No. 5-077884 Korean Published Patent Publication No. 2003-0052248 Japanese Published Patent Publication No. 10-158783 Japanese Published Patent Publication No. 6-057336 Japanese Published Patent Publication No. 9-227951 Japanese Published Patent Publication No. 10-265900 Japanese Published Patent Publication No. Hei 6-240365 Japanese Published Patent Publication No. 7-216340 Japanese Published Patent Publication No. 10-280048 Japanese Published Patent Publication No. Hei 10-287954

  An object of the present invention is to provide a Nb and Ti composite-added IF cold-rolled steel sheet that can reduce the in-plane anisotropy index as well as a high yield ratio, and a method for producing the same.

The cold-rolled steel sheet of the present invention is, by weight, C: 0.01% or less, Cu: 0.01-0.2%, S: 0.005-0.08%, Al: 0.1% or less, Including N: 0.004% or less, P: 0.2% or less, B: 0.0001-0.002%, Nb: 0.002-0.04%, Ti: 0.005-0.15% , Composed of the remaining Fe and other inevitable impurities,
1 ≦ (Cu / 63.5) / (S ^★ / 32) ≦ 30,
S ^* = S-0.8x (Ti-0.8x (48/14) xN) x (32/48) is satisfied, and the average size of the CuS precipitate is 0.2 μm or less.

The cold-rolled steel sheet of the present invention is, by weight, C: 0.01% or less, Cu: 0.01-0.2%, Mn: 0.01-0.3%, S: 0.005-0. 08%, Al: 0.1% or less, N: 0.004% or less, P: 0.2% or less, B: 0.0001-0.002%, Nb: 0.002-0.04%, Ti : 0.005-0.15% inclusive, composed of remaining Fe and other inevitable impurities,
1 ≦ (Mn / 55 + Cu / 63.5) / (S ^★ / 32) ≦ 30,
S ^* = S-0.8x (Ti-0.8x (48/14) xN) x (32/48) is satisfied, and the average size of the (Mn, Cu) S precipitate is 0.2 μm or less. Will come to do.

The cold-rolled steel sheet of the present invention is, by weight, C: 0.01% or less, Cu: 0.01-0.2%, S: 0.005-0.08%, Al: 0.1% or less, N: 0.004-0.02%, P: 0.2% or less, B: 0.0001-0.002%, Nb: 0.002-0.04%, Ti: 0.005-0.15 %, And is composed of the remaining Fe and other inevitable impurities,
1 ≦ (Cu / 63.5) / (S ^★ / 32) ≦ 30,
1 ≦ (Al / 27) / (N ^* / 14) ≦ 10,
S ^* = S-0.8x (Ti-0.8x (48/14) xN) x (32/48), N ^* = N-0.8x (Ti-0.8x (48/32) xS) x (14/48) is satisfied, and the average size of CuS precipitates and AlN precipitates is distributed at 0.2 μm or less.

The cold-rolled steel sheet of the present invention is, by weight, C: 0.01% or less, Cu: 0.01-0.2%, Mn: 0.01-0.3%, S: 0.005-0. 08%, Al: 0.1% or less, N: 0.004-0.02%, P: 0.2% or less, B: 0.0001-0.002%, Nb: 0.002-0.04 %, Ti: 0.005-0.15%, composed of the remaining Fe and other inevitable impurities,
1 ≦ (Mn / 55 + Cu / 63.5) / (S ^★ / 32) ≦ 30,
1 ≦ (Al / 27) / (N ^* / 14) ≦ 10,
S ^* = S-0.8x (Ti-0.8x (48/14) xN) x (32/48), N ^* = N-0.8x (Ti-0.8x (48/32) xS) x (14/48) is satisfied, and the average size of (Mn, Cu) S precipitate and AlN precipitate is distributed at 0.2 μm or less.

The cold-rolled steel sheet of the present invention is, by weight, C: 0.01% or less, S: 0.08% or less, Al: 0.1% or less, N: 0.004% or less, P: 0.2% Hereinafter, B: 0.0001-0.002%, Nb: 0.002-0.04%, Ti: 0.005-0.15%, where Cu: 0.01-0.2%, Mn: Containing one or more of 0.01-0.3%, N: 0.004-0.2%, composed of the remaining Fe and other inevitable impurities,
1 ≦ (Mn / 55 + Cu / 63.5) / (S ^★ / 32) ≦ 30,
1 ≦ (Al / 27) / (N ^* / 14) ≦ 10 (provided that the content of N is 0.004% or more),
S ^* = S-0.8x (Ti-0.8x (48/14) xN) x (32/48), N ^* = N-0.8x (Ti-0.8x (48/32) xS) x (14/48) is satisfied, and at least one of (Mn, Cu) S precipitate and AlN precipitate is distributed with an average size of 0.2 μm or less.

In the cold-rolled steel sheet of the present invention, the contents of the C, Ti, Nb, N, and S are as follows:
If satisfy ^{0.8 ≦ (Ti ★ /48+Nb/93)/(C/12)≦5.0,Ti ★ =} Ti-0.8x ((48/14) xN + (48/32) xS), room temperature Has non-aging characteristics. Cs (solute carbon) determined by C and Ti is 5-30 [where Cs = (C-Nbx12 / 93-Ti ^* x12 / 48) x10000, Ti ^* = Ti-0.8x (( 48/14) xN + (48/32) xS) However, when Ti ^* <0, Ti ^* = 0 is satisfied, and bake-hardening characteristics are obtained.

  The cold-rolled steel sheet of the present invention has characteristics of a soft cold-rolled steel sheet of 280 MPa class and a high-strength cold-rolled steel sheet of 340 MPa or more by component design.

  In the above component system, when the P content is 0.015% or less, a 280 MPa grade soft cold-rolled steel sheet is obtained. If this cold-rolled steel sheet further includes one or two of solid solution strengthening elements Si and Cr, or a P content of 0.015-0.2%, a high strength characteristic of 340 MPa or more is secured. . In the case of high strength steel containing P alone, the P content is preferably 0.03-0.2%. In the case of Si, 0.1-0.8% is preferable, and in the case of Cr, 0.2-1.2% is preferable. When one or more of Si and Cr are included, the P content can be variously designed within a range of 0.2% or less.

  If workability is to be further improved in the cold-rolled steel sheet of the present invention, Mo may further be included in an amount of 0.01 to 0.2%.

The manufacturing method of the above-mentioned cold-rolled steel sheet is obtained by reheating a slab satisfying the above-described component system of the present invention at a temperature of 1100 ° C. or higher, then hot rolling the finishing rolling temperature to be higher than the Ar ₃ transformation point. Cooling at a rate of 300 ° C./min or more, winding at a temperature of 700 ° C. or less, cold rolling, and continuous annealing.

  As described above, the present invention refines the crystal grains by distributing fine precipitates in the Nb-Ti composite IF steel, thereby reducing the in-plane anisotropy index, and increasing the yield strength by precipitation strengthening. It is to improve.

  Hereinafter, the present invention will be described in detail.

  In the cold-rolled steel sheet of the present invention, fine precipitates of 0.2 μm or less are distributed. This precipitate includes MnS, CuS, or a composite precipitate of MnS and CuS. The precipitate is simply labeled (Mn, Cu) S.

  The present invention is a composite addition of Nb and Ti (simply described as Nb-Ti composite system) IF steel, when precipitates are finely distributed, yield strength is enhanced, in-plane anisotropy index is lowered, and workability is improved. It was completed based on the research results. Precipitates used in the present invention were not noticed in IF steel. Furthermore, these precipitates were not actively used in terms of increasing yield strength and in-plane anisotropy.

  In order to secure (Mn, Cu) S precipitates and / or AlN precipitates in Nb-Ti composite IF steel, component management is required. When Ti, Zr, etc. are contained in IF steel, S and N react preferentially with Ti, Zr. Since the cold-rolled steel sheet of the present invention is Nb-Ti composite IF steel, Ti reacts with C, N, and S. Therefore, it is necessary to manage various components so that S is (Mn, Cu) S and N is precipitated with AlN.

  The fine precipitate obtained by the present invention makes the crystal grains fine. As the crystal grains become finer, the grain boundary ratio becomes relatively large. Therefore, more solute carbon is present in the crystal grain boundary than in the crystal grain, and normal temperature non-aging characteristics are ensured. Since the solid solution carbon remaining in the crystal grains is more freely moved, it is combined with the movable dislocation and affects the aging characteristics at room temperature. On the other hand, solute carbon that segregates at a more stable position, such as around crystal grain boundaries and precipitates, is activated at a high temperature such as a coating baking process and affects the bake hardening characteristics.

  The finely distributed precipitates according to the present invention have a positive effect on the increase in yield strength and strength-ductility balance characteristics due to precipitation strengthening, as well as in-plane anisotropy and plastic anisotropy. For this purpose, (Mn, Cu) S precipitates and AlN precipitates must be finely distributed. According to the present invention, the fine distribution of the precipitates described above has the greatest influence on the content of the components affecting the precipitates, the component ratio conditions, and the manufacturing conditions, particularly the cooling rate after hot rolling is finished.

  First, the components of the cold-rolled steel sheet of the present invention will be described.

  The content of carbon (C) is preferably 0.01% or less. Carbon affects the aging resistance at normal temperature and the bake hardening characteristics. When the carbon content exceeds 0.01%, expensive Nb and Ti are further added to remove carbon, which is not preferable in terms of economy and not preferable in terms of moldability. If only room temperature aging resistance is desired, it is preferable to control the carbon content in a low range. This can reduce the amount of expensive Nb and Ti added. If it is intended to ensure the bake-hardening properties, it is preferable to add 0.001% or more, more preferably 0.005% -0.01%. When the carbon content is relatively 0.005% or less, normal temperature aging characteristics can be ensured without increasing the amount of expensive Nb and Ti added.

  The content of copper (Cu) is preferably 0.01-0.2%. Copper forms fine CuS precipitates, makes crystal grains fine, lowers the in-plane anisotropy index, and increases yield strength by precipitation strengthening. For this purpose, fine Cu cannot be precipitated unless the Cu content is 0.01% or more, and coarse precipitation occurs if it exceeds 0.2%. A more preferable Cu content is 0.03-0.2%.

  The content of manganese (Mn) is preferably 0.01 to 0.3%. Manganese is known as a solid solution strengthening element by precipitating sulfur in a solid solution state in steel with MnS to prevent red short brittleness (hot shortness) due to solid solution sulfur. From such a technical point of view, it is common to add a high manganese content. However, in the present invention, the manganese content is made 0.3% or less based on the research result that MnS is deposited very finely when the sulfur content becomes appropriate while the manganese content is lowered. In order to ensure such characteristics, the manganese content must be 0.01% or more. When the content of manganese is less than 0.01%, the content of sulfur remaining in a solid solution state is large, so that red heat embrittlement may occur. If the manganese content exceeds 0.3%, the manganese content is high and coarse MnS precipitates are produced, making it difficult to ensure strength. More preferably, the Mn content is 0.01-0.12%.

  The content of sulfur (S) is preferably 0.08% or less. Sulfur (S) reacts with Cu or / and Mn to form fine CuS and MnS precipitates. When the sulfur content exceeds 0.08%, the dissolved sulfur content is large, the ductility and formability are very low, and red hot brittleness may occur. When positively obtaining CuS and / or MnS precipitates, the sulfur content is preferably 0.005% or more.

  The aluminum (Al) content is preferably 0.1% or less. Aluminum reacts with N to form fine AlN precipitates and completely prevents aging by solute nitrogen. When the nitrogen content is 0.004% or more, sufficient AlN precipitates are secured, and when these precipitates are finely distributed, the yield strength is increased by precipitation strengthening as well as refinement of crystal grains. A more preferable Al content is 0.01-0.1%.

  The content of nitrogen (N) is preferably 0.02% or less. In the case of nitrogen, when using the precipitate of AlN, it is added to 0.02%, otherwise it is controlled to 0.004% or less. When the N content is less than 0.004%, the number of precipitated AlN is small, and the effect of crystal grain refinement and precipitation strengthening is small. When the nitrogen content exceeds 0.02%, it is difficult to guarantee aging with solid nitrogen.

  The phosphorus (P) content is preferably 0.2% or less. Phosphorus is an element having a high solid solution strengthening effect and a small decrease in the r value, and ensures high strength in the steel for controlling precipitates according to the present invention. The steel content is required to have a strength of 280 Mpa grade, and the P content is preferably 0.015% or less. For high-strength steel of 340 Mpa grade or higher, it is better to exceed 0.015-0.2%. When the P content exceeds 0.2%, the ductility is lowered and the upper limit is preferably limited to 0.2%. When Si and Cr are added in the present invention, various strength designs can be made while keeping the P content in the range of 0.2% or less.

  The content of boron (B) is preferably 0.0001-0.002%. Boron is added to prevent secondary work brittleness. For this reason, the boron content is preferably 0.0001% or more. If the boron content exceeds 0.002%, deep drawing may be greatly reduced.

  The content of niobium (Nb) is preferably 0.002 to 0.04%. Nb is added for the purpose of ensuring non-aging properties and improving moldability. Nb is a strong carbide-forming element and is added to steel to precipitate NbC precipitates. NbC precipitates have the effect of developing a texture during annealing and greatly improving deep drawability. When the amount of Nb added is 0.002% or less, the amount of NbC precipitates is so small that there is little development of the texture and there is almost no effect of improving deep drawing workability. When Nb exceeds 0.04%, the amount of NbC precipitates is too large, deep drawing workability and elongation rate are lowered, and moldability may be greatly reduced.

  The content of titanium (Ti) is 0.005-0.15%. Titanium is added for the purpose of securing non-aging properties and improving formability, but titanium is a strong carbide-forming element and is added to steel to precipitate non-aging by depositing TiC precipitates and solid solution carbon. Ensure sex. When the amount of titanium added is less than 0.005%, the amount of TiC precipitates is so small that there is little effect of improving deep drawing workability with little texture development. When Ti exceeds 0.15%, the size of the TiC precipitate is too large, the grain refinement effect is reduced, the in-plane anisotropy index is increased, the yield strength is lowered, and the plating characteristics are greatly lowered. .

  In the present invention, the contents of Mn, Cu, S, Nb, Ti, Al, N, and C are managed as follows in order to secure (Mn, Cu) S and AlN precipitates. In the following relational expression, each component is used in% by weight.

[Relational expression 1]
1 ≦ (Cu / 63.5) / (S ^★ / 32) ≦ 30

[Relational expression 2]
S ^* = S-0.8x (Ti-0.8x (48/14) xN) x (32/48)

In relational expression 1, S ^* is the content of S which does not react with Ti but remains and reacts with Cu. S ^* is determined by relational expression 2. In order to secure fine CuS precipitates, the value of relational expression 1 is preferably 1 or more. When the value of the relational expression 1 exceeds 30, coarse CuS precipitates are distributed, which is not preferable. In order to stably secure CuS of 0.2 μm or less, a preferable value of the relational expression 1 is 1-20, more preferably 1-9, and most preferably 1-6.

[Relational expression 3]
1 ≦ (Mn / 55 + Cu / 63.5) / (S ^★ / 32) ≦ 30

  Relational expression 3 is for securing a precipitate of (Mn, Cu) S, and Mn is added to relational expression 1. If the value of the relational expression 3 is not 1 or more, an effective (Mn, Cu) S precipitate cannot be obtained, and if it exceeds 30, the (Mn, Cu) S precipitate becomes coarse. In order to stably secure (Mn, Cu) S of 0.2 μm or less, a preferable value of the relational expression 3 is 1-20, more preferably 1-9, and most preferably 1-6. When both Mn and Cu are added, the sum of Mn and Cu is more preferably 0.05-0.4%. This is to obtain fine (Mn, Cu) S precipitates.

[Relational expression 4]
1 ≦ (Al / 27) / (N ^★ / 14) ≦ 10

[Relational expression 5]
N ^* = N-0.8x (Ti-0.8x (48/32) xS) x (14/48)

Relational expression 4 is for ensuring fine AlN precipitates. In relational expression 4, N ^* is the total N content which does not react with Ti but remains and reacts with Al. N ^* is determined by relational expression 5. In order to ensure fine AlN precipitates, it is preferable that the value of relational expression 4 satisfies 1-10. If the value of the relational expression 4 is not 1 or more, an effective AlN precipitate cannot be precipitated, and if it exceeds 10, the AlN precipitate is coarse and the workability and yield strength characteristics are not good. More preferably, the value of relational expression 4 satisfies 1-6.

The cold-rolled steel sheet of the present invention can be variously combined depending on the type of precipitate to be obtained. That is, by weight, C: 0.01% or less, S: 0.08% or less, Al: 0.1% or less, N: 0.004% or less, P: 0.2% or less, B: 0.00. 0001-0.002%, Nb: 0.002-0.04%, Ti: 0.005-0.15%, where Cu: 0.01-0.2%, Mn: 0.01-0. 1% or more of 3%, N: 0.004-0.2%, composed of the remaining Fe and other inevitable impurities,
1 ≦ (Mn / 55 + Cu / 63.5) / (S ^★ / 32) ≦ 30,
1 ≦ (Al / 27) / (N ^* / 14) ≦ 10 (provided that the content of N is 0.004% or more),
S ^* = S-0.8x (Ti-0.8x (48/14) xN) x (32/48), N ^* = N-0.8x (Ti-0.8x (48/32) xS) x (14/48) is satisfied, and at least one kind of MnS, CuS, a composite precipitate of MnS and CuS, and an AlN precipitate of 0.2 μm or less comes to exist. That is, when at least one kind or two or more kinds are selected from the group of Cu: 0.01-0.2%, Mn: 0.01-0.3%, N: 0.004-0.2%, Various combinations of (Mn, Cu) S and AlN precipitates of 0.2 μm or less can be obtained.

  In the present invention, carbon is deposited with NbC and TiC. Therefore, normal temperature aging characteristics and bake hardening characteristics are affected by the conditions of solute carbon not precipitated by NbC and TiC. Considering this, it is most preferable that Nb, Ti and C satisfy the following conditions.

[Relational expression 6]
0.8 ≦ (Ti ^{★ /} 48 + Nb / 93) / (C / 12) ≦ 5.0,

[Relational expression 7]
Ti ^* = Ti-0.8x ((48/14) xN + (48/32) xS)

Relational expression 6 is for precipitating NbC and TiC to remove carbon in a solid solution state, thereby ensuring normal temperature non-aging characteristics. In relational expression 6, Ti ^* is the content of Ti that reacts with N and S and reacts with C from the total Ti content. Ti ^* is determined by relational expression 7.

  When the value of the relational expression 6 is less than 0.8, it is difficult to ensure the normal temperature non-aging characteristics. When the value of the relational expression 6 exceeds 5, the Nb and Ti remaining in a solid solution state in the steel The amount of is increased and ductility is lowered. In order to ensure the room temperature non-aging characteristics without securing the bake hardening characteristics, it is preferable to manage the carbon content to 0.005% or less. If the relational expression 6 is satisfied when the carbon content exceeds 0.005%, the non-aging property at room temperature can be ensured, but the Nb and TiC precipitates increase and the workability may decrease.

[Relational expression 8]
Cs = (C-Nbx12 / 93-Ti ^* x12 / 48) x10000
(However, if Ti ^★ <0, Ti ^★ = 0)

  Relational expression 8 is for ensuring bake hardening characteristics, and Cs indicates the content of solid solution carbon not precipitated by NbC and TiC. Cs is calculated by the relational expression 8, and the unit of the value is ppm. If the Cs value is not 5 ppm or more, the bake hardening amount cannot be ensured, and if it exceeds 30 ppm, the content of solid solution carbon is high and it is difficult to ensure non-aging at room temperature.

  In the component system of the present invention, the precipitate is more advantageous as it is finely distributed, but preferably the average size is 0.2 μm or less. According to the research results of the present invention, when the average size of the precipitates exceeds 0.2 μm, the strength is particularly low and the in-plane anisotropy index is not good.

Furthermore, although a large amount of precipitates of 0.2 μm or less is distributed in the component system of the present invention, the distribution number is not particularly limited, but the higher the distribution number, the more advantageous. Preferably, the number of precipitates distributed is 1 × 10 ⁵ or more per mm ² , more preferably 1 × 10 ⁶ or more per mm ² , and most preferably 1 × 10 ⁷ or more per mm ² . The greater the number of precipitate distributions, the higher the plastic anisotropy index and the lower the in-plane anisotropy index, so that the workability is greatly improved. In general, it is known that as the plastic anisotropy index increases, the in-plane anisotropy index increases and there is a limit to increasing the plastic anisotropy index in terms of workability. It is worth noting the unique change that the plastic anisotropy index increases and the in-plane anisotropy index decreases according to the number of distributions of the precipitates of the present invention. When fine precipitates are obtained by the present invention, the yield ratio (yield strength / tensile strength) satisfies 0.58 or more.

  In the present invention, when applied to a high-strength steel sheet of 340 MPa class or higher, a solid solution strengthening element such as P, that is, one or more of P, Si, and Cr can be added. Since P has been described above, repeated description is omitted.

  The silicon (Si) content is preferably 0.1-0.8%. Si is an element having a high solid solution strengthening effect and a low decrease in elongation rate, and guarantees high strength in steel for controlling precipitates according to the present invention. If the Si content is not more than 0.1%, the strength cannot be ensured, and if it exceeds 0.8%, the ductility decreases.

  The content of chromium (Cr) is preferably 0.2 to 1.2%. Cr is an element that has a high solid solution strengthening effect, lowers the secondary work brittle temperature, and lowers the aging index by Cr carbide. By the present invention, steel that controls precipitates ensures high strength and in-plane anisotropy. Lower the index. If the Cr content is not 0.2% or more, the strength cannot be secured, and if it exceeds 1.2%, the ductility is lowered.

  Molybdenum (Mo) can be further added to the cold-rolled steel sheet of the present invention.

  The content of molybdenum (Mo) is preferably 0.01 to 0.2%. Mo is added as an element that increases the plastic anisotropy index. However, if the content is not more than 0.01%, the plastic anisotropy index does not increase. There is a risk of causing hot brittleness without increasing the height.

[Method for producing cold-rolled steel sheet]
Below, the manufacturing method of the cold rolled sheet steel of this invention is demonstrated through the most preferable embodiment. The embodiment of the present invention described here can be modified in various forms, and the scope of the present invention is not limited to the embodiment described below.

  The present invention is characterized in that a steel satisfying the above steel composition is subjected to hot rolling and cold rolling so that the average size of precipitates is 0.2 μm or less on the cold rolled sheet. The average size of the precipitates in the cold rolled sheet is affected by the manufacturing process such as the reheating temperature and the coiling temperature as well as the component design, but is particularly directly affected by the cooling rate after hot rolling.

[Hot rolling conditions]
In the present invention, steel satisfying the above steel composition is reheated and hot rolled. The reheating temperature is preferably 1100 ° C. or higher. When the reheating temperature is less than 1100 ° C., the reheating temperature is low, and coarse precipitates generated during continuous casting remain in a state where they are not completely dissolved, and there are many coarse precipitates even after hot rolling. This is because it remains.

The hot rolling is preferably performed under the condition that the finishing rolling temperature is equal to or higher than the Ar ₃ transformation temperature. This is because when the finishing rolling temperature is lower than the Ar ₃ transformation temperature, not only the workability is lowered due to the formation of rolled grains, but also the strength is lowered.

  After hot rolling, the cooling rate before winding is preferably 300 ° C./min or more. Even if the component ratio is controlled to obtain fine precipitates according to the present invention, if the cooling rate is less than 300 ° C./min, the average size of the precipitates may exceed 0.2 μm. That is, as the cooling rate increases, more nuclei are generated and the precipitates become finer. There is no need to limit the upper limit of the cooling rate because the size of the precipitate becomes finer as the cooling rate becomes faster. However, even if the cooling rate becomes higher than 1000 ° C./min, the effect of refining the precipitate is larger than this. Therefore, the cooling rate is more preferably 300 to 1000 ° C./min.

[Winding condition]
Winding is performed after hot rolling as described above, and the winding temperature is preferably 700 ° C. or lower. When the coiling temperature exceeds 700 ° C., the precipitate grows very coarsely and it is difficult to ensure the strength.

[Cold rolling conditions]
Cold rolling is preferably performed at a rolling reduction of 50-90%. When the cold rolling reduction is less than 50%, the amount of nucleation of annealing recrystallization is small, so that the crystal grains grow very large during annealing, and the strength and formability decrease due to the coarsening of the annealing recrystallization grains. When the cold reduction ratio exceeds 90%, the formability is improved, but the nucleation amount is too much, and the annealed recrystallized grains are rather fine and the ductility is lowered.

[Continuous annealing]
The continuous annealing temperature plays an important role in determining the product material. In this invention, it is preferable to carry out in the temperature range of 700-900 degreeC. When the continuous annealing temperature is less than 700 ° C., recrystallization is not completed, and thus the target ductility value cannot be secured. When the annealing temperature exceeds 900 ° C., the strength decreases due to coarsening of the recrystallized grains. The continuous annealing time is maintained so that the recrystallization is completed, but the recrystallization is completed if it is about 10 seconds or longer. Preferably, the continuous annealing time is within a range of 10 seconds to 30 minutes.

  Hereinafter, the present invention will be described more specifically through examples.

In the examples, the mechanical properties were processed with standard specimens according to the ASTM standard (ASTM E-8 standard), and then yield strength, tensile strength, elongation, plastic anisotropy using a tensile tester (INSTRON, Model 6025). sex index (r _m value), the in-plane anisotropy index (△ r value) and were measured aging evaluation index. Here, r _m = (r ₀ + 2r ₄₅ + r ₉₀ ) / 4, Δr = (r ₀ -2r ₄₅ + r ₉₀ ) / 2.

  The aging evaluation index is a yield point elongation rate measured after a 1.0% skin pass rolling after annealing and after heat treatment of the specimen at 100 ° C. for 2 hours.

  Bake-hardening characteristics are obtained by measuring the yield strength after heat treatment at 170 ° C. for 20 minutes after adding 2% strain to the specimen, and subtracting the yield strength value before heat treatment from the measured yield strength value to make the BH value. It is.

[ Reference Example 1 ]
The steel slab satisfying the chemical composition shown in the following table is reheated, finished hot rolled, cooled at a rate of 400 ° C./min, wound at 650 ° C., and then cold rolled at a reduction rate of 75%. Continuous annealing was performed. The finishing rolling temperature at this time was 910 ° C. above the Ar 3 transformation point, and the continuous annealing was heated at 830 ° C. for 40 seconds at a rate of 10 ° C./second to produce a cold-rolled steel sheet.

[Example 2]
The steel slab satisfying the chemical composition shown in the following table is reheated, finished hot rolled, cooled at a rate of 400 ° C./min, wound at 650 ° C., and then cold rolled at a reduction rate of 75%. Continuous annealing was performed. The finishing rolling temperature at this time was 910 ° C. above the Ar 3 transformation point, and the continuous annealing was heated at 830 ° C. for 40 seconds at a rate of 10 ° C./second to produce a cold-rolled steel sheet.

[ Reference Example 2 ]
The steel slab satisfying the chemical composition shown in the table below is reheated, finished hot rolled, cooled at a rate of 400 ° C./min, wound at 650 ° C., and then cold rolled at a reduction rate of 75%. A cold-rolled steel sheet was manufactured by continuous annealing. The finishing rolling temperature at this time was 910 ° C. above the Ar 3 transformation point, and the continuous annealing was performed by heating at 830 ° C. for 40 seconds at a rate of 10 ° C./second.

[Example 4]
The steel slab satisfying the chemical composition shown in the table below is reheated, finished hot rolled, cooled at a rate of 400 ° C./min, wound at 650 ° C., and then cold rolled at a reduction rate of 75%. A cold-rolled steel sheet was manufactured by continuous annealing. The finishing rolling temperature at this time was 910 ° C. above the Ar 3 transformation point, and the continuous annealing was heated at 830 ° C. for 40 seconds at a rate of 10 ° C./second.

[ Reference Example 3 ]
The steel slab satisfying the chemical composition shown in the table below is reheated, finished hot rolled, cooled at a rate of 400 ° C./min, wound at 650 ° C., and then cold rolled at a reduction rate of 75%. A cold-rolled steel sheet was manufactured by continuous annealing. The finishing rolling temperature at this time was 910 ° C. above the Ar 3 transformation point, and continuous annealing was performed at 830 ° C. for 40 seconds at a rate of 10 ° C./second.

[ Reference Example 4 ]
The steel slab satisfying the chemical composition shown in the following table is reheated, finished hot rolled, cooled at a rate of 400 ° C./min, wound at 650 ° C., and then cold rolled at a reduction rate of 75%. A cold-rolled steel sheet was manufactured by continuous annealing. The finishing rolling temperature at this time was 910 ° C. above the Ar 3 transformation point, and continuous annealing was performed at 830 ° C. for 40 seconds at a rate of 10 ° C./second.

[ Reference Example 5 ]
The steel slab satisfying the chemical composition shown in the table below is reheated, finished hot rolled, cooled at a rate of 400 ° C./min, wound at 650 ° C., and then cold rolled at a reduction rate of 75%. A cold-rolled steel sheet was manufactured by continuous annealing. The finishing rolling temperature at this time was 910 ° C. above the Ar 3 transformation point, and continuous annealing was performed at 830 ° C. for 40 seconds at a rate of 10 ° C./second.

[ Reference Example 6 ]
The steel slab satisfying the chemical composition presented in the table below is reheated, hot rolled for finishing, cooled at a rate of 400 ° C / min, wound at 650 ° C, and then cooled at a reduction rate of 75%. Hot rolling and continuous annealing were performed. The finishing rolling temperature at this time was 910 ° C. above the Ar 3 transformation point, and the continuous annealing was heated at 830 ° C. for 40 seconds at a rate of 10 ° C./second to produce a specimen.

[Example 9]
The steel slab satisfying the chemical composition shown in the following table is reheated, finished hot rolled, cooled at a rate of 400 ° C./min, wound at 650 ° C., and then cold rolled at a reduction rate of 75%. A cold-rolled steel sheet was manufactured by continuous annealing. The finishing rolling temperature at this time was 910 ° C. above the Ar 3 transformation point, and continuous annealing was performed at 830 ° C. for 40 seconds at a rate of 10 ° C./second.

[ Reference Example 7 ]
The steel slab satisfying the chemical composition shown in the table below is reheated, finished hot rolled, cooled at a rate of 400 ° C./min, wound at 650 ° C., and then cold rolled at a reduction rate of 75%. Continuous annealing was performed. The finishing rolling temperature at this time was 910 ° C. above the Ar 3 transformation point, and continuous annealing was performed at 830 ° C. for 40 seconds at a rate of 10 ° C./second.

[Example 11]
The steel slab satisfying the chemical composition shown in the following table is reheated, finished hot rolled, cooled at a rate of 400 ° C./min, wound at 650 ° C., and then cold rolled at a reduction rate of 75%. A cold-rolled steel sheet was manufactured by continuous annealing. The finishing rolling temperature at this time was 910 ° C. above the Ar 3 transformation point, and continuous annealing was performed at 830 ° C. for 40 seconds at a rate of 10 ° C./second.

[ Reference Example 8 ]
The steel slab satisfying the chemical composition shown in the following table is reheated, finished hot rolled, cooled at a rate of 400 ° C./min, wound at 650 ° C., and then cold rolled at a reduction rate of 75%. Continuous annealing was performed. The finishing rolling temperature at this time was 910 ° C. above the Ar 3 transformation point, and continuous annealing was performed at 830 ° C. for 40 seconds at a rate of 10 ° C./second.

[ Reference Example 9 ]
The steel slab satisfying the chemical composition shown in the following table is reheated, finished hot rolled, cooled at a rate of 400 ° C./min, wound at 650 ° C., and then cold rolled at a reduction rate of 75%. Continuous annealing was performed. The finishing rolling temperature at this time was 910 ° C. above the Ar 3 transformation point, and continuous annealing was performed at 830 ° C. for 40 seconds at a rate of 10 ° C./second.

[ Reference Example 10 ]
The steel slab satisfying the chemical composition shown in the following table is reheated, finished hot rolled, cooled at a rate of 400 ° C./min, wound at 650 ° C., and then cold rolled at a reduction rate of 75%. Continuous annealing was performed. The finishing rolling temperature at this time was 910 ° C. above the Ar 3 transformation point, and continuous annealing was performed at 830 ° C. for 40 seconds at a rate of 10 ° C./second.

  In the present invention, the embodiment is merely an example, and the present invention is not limited thereto. Any device that has substantially the same configuration as the technical idea described in the claims of the present invention and exhibits the same effects can be included in the technical scope of the present invention.

Claims (36)

% By weight, C: 0.01% or less, Cu: 0.01-0.2%, S: 0.005-0.08%, Al: 0.1% or less, N: 0.02% or less, P: 0.2% or less, B: 0.0001-0.002%, Nb: 0.002-0.04%, Ti: 0.005-0.15%, Mn: 0.01-0.3 %, And is composed of the remaining Fe and other inevitable impurities,
1 ≦ (Mn / 55 + Cu / 63.5) / (S ^★ / 32) ≦ 30,
1 ≦ (Cu / 63.5) / (S ^★ / 32) ≦ 30,
S ^* = S-0.8 * (Ti-0.8 * (48/14) * N) * (32/48)
(Mn, Cu) A high yield ratio cold-rolled steel sheet having excellent formability with an average size of S precipitates of 0.2 μm or less. When the content of N is 0.004-0.02%,
1 ≦ (Al / 27) / (N ^* / 14) ≦ 10,
N ^* = N−0.8 * (Ti−0.8 * (48/32) * S) * (14/48) is satisfied, and the average size of the AlN precipitates is 0.2 μm or less. The high yield ratio cold-rolled steel sheet having excellent formability according to claim 1. % By weight, C: 0.01% or less, S: 0.08% or less, Al: 0.1% or less, N: 0.02% or less, P: 0.2% or less, B: 0.0001- Including 0.002%, Nb: 0.002-0.04%, Ti: 0.005-0.15%, Cu: 0.01-0.2%, Mn: 0.01-0.3% , Composed of the remaining Fe and other inevitable impurities,
1 ≦ (Mn / 55 + Cu / 63.5) / (S ^★ / 32) ≦ 30,
1 ≦ (Al / 27) / (N ^* / 14) ≦ 10 (provided that the content of N is 0.004% or more),
S ^* = S-0.8 * (Ti-0.8 * (48/14) * N) * (32/48), N ^* = N-0.8 * (Ti-0.8 * (48 / 32) * S) * (14/48) is satisfied, and the high yield is excellent in formability in which the average size of at least one of (Mn, Cu) S precipitate and AlN precipitate is 0.2 μm or less. Specific cold rolled steel sheet. The content of C, Ti, Nb, N, S is the following relationship:
Satisfy ^{0.8 ≦ (Ti ★ /48+Nb/93)/(C/12)≦5.0,Ti ★ =} Ti-0.8 * ((48/14) * N + (48/32) * S) The high yield ratio cold-rolled steel sheet having excellent formability according to claim 1 or 3.   The high yield ratio cold-rolled steel sheet having excellent formability according to claim 4, wherein the C content is 0.005% or less. Wherein C is 5-30 [where Cs (solute carbon) which is determined by the Ti, Cs = (C-Nb * 12/93-Ti ★ * 12/48) * 10000, Ti ★ = Ti-0.8 * ((48/14) * N + (48/32) * S) However, when Ti ^* <0, Ti ^* = 0 is satisfied. High yield ratio cold rolled steel sheet with excellent formability.   The high yield ratio cold-rolled steel sheet having excellent formability according to claim 6, wherein the C content is 0.001 to 0.01%.   The high yield ratio cold-rolled steel sheet having excellent formability according to any one of claims 1 to 3, wherein a yield ratio (yield strength / tensile strength) is 0.58 or more. The high yield ratio cold-rolled steel sheet having excellent formability according to any one of claims 1 to 3, wherein the number of precipitates is 1 * 10 ⁶ pieces / mm ² or more.   The high yield ratio cold-rolled steel sheet having excellent formability according to claim 1 or 3, wherein the P content is 0.015% or less.   The high yield ratio cold-rolled steel sheet having excellent formability according to claim 1 or 3, wherein the P content is 0.03-0.2%.   Furthermore, 1 type or 2 types of Si: 0.1-0.8% and Cr: 0.2-1.2% are contained, It is excellent in the moldability of Claim 1 or Claim 3 characterized by the above-mentioned. High yield ratio cold rolled steel sheet.   Furthermore, Mo is contained in 0.01-0.2%, The high yield ratio cold-rolled steel plate excellent in formability of Claim 1 or Claim 3 characterized by the above-mentioned.   Furthermore, Mo is contained in 0.01-0.2%, The high yield ratio cold-rolled steel plate excellent in formability of Claim 12.   The high yield ratio cold-rolled steel sheet having excellent formability according to claim 3, wherein the sum of Mn and Cu is 0.05 to 0.4%.   The high yield ratio cold-rolled steel sheet having excellent formability according to claim 3, wherein the Mn content is 0.01-0.12%. The high yield ratio cold-rolled steel sheet with excellent formability according to claim 3, wherein (Mn / 55 + Cu / 63.5) / (S ^* / 32) satisfies 1-9. The high yield ratio cold-rolled steel sheet having excellent formability according to claim 2 or 3, wherein (Al / 27) / (N ^* / 14) satisfies 1-6. % By weight, C: 0.01% or less, Cu: 0.01-0.2%, S: 0.005-0.08%, Al: 0.1% or less, N: 0.02% or less, P: 0.2% or less, B: 0.0001-0.002%, Nb: 0.002-0.04%, Ti: 0.005-0.15%, Mn: 0.01-0.3 %, And is composed of the remaining Fe and other inevitable impurities,
1 ≦ (Mn / 55 + Cu / 63.5) / (S ^★ / 32) ≦ 30,
1 ≦ (Cu / 63.5) / (S ^★ / 32) ≦ 30,
Reheat the slab satisfying S ^* = S-0.8 * (Ti-0.8 * (48/14) * N) * (32/48) at a temperature of 1100 [deg.] C. or higher, and then set the finishing rolling temperature. Hot-rolled at an Ar ₃ transformation point or higher, cooled at a rate of 300 ° C./min or higher, wound at a temperature of 700 ° C. or lower, cold-rolled, and continuously annealed to have an average size of 0.2 μm. The manufacturing method of the high yield ratio cold-rolled steel plate excellent in the formability in which the following (Mn, Cu) S precipitates are distributed. When the content of N is 0.004-0.02%, 1 ≦ (Al / 27) / (N ^* / 14) ≦ 10,
N ^* = N−0.8 * (Ti−0.8 * (48/32) * S) * (14/48) is satisfied, and the average size of the AlN precipitates is 0.2 μm or less. The manufacturing method of the high yield ratio cold-rolled steel plate excellent in the formability of Claim 19. % By weight, C: 0.01% or less, S: 0.08% or less, Al: 0.1% or less, N: 0.02% or less, P: 0.2% or less, B: 0.0001- Including 0.002%, Nb: 0.002-0.04%, Ti: 0.005-0.15%, Cu: 0.01-0.2%, Mn: 0.01-0.3% , Composed of the remaining Fe and other inevitable impurities,
1 ≦ (Mn / 55 + Cu / 63.5) / (S ^★ / 32) ≦ 30,
1 ≦ (Al / 27) / (N ^* / 14) ≦ 10 (provided that the content of N is 0.004-0.02%),
S ^* = S-0.8 * (Ti-0.8 * (48/14) * N) * (32/48), N ^* = N-0.8 * (Ti-0.8 * (48 / 32) A slab satisfying * S) * (14/48)) is reheated at a temperature of 1100 ° C. or higher and then hot rolled at a finishing rolling temperature of Ar ₃ transformation point or higher and a speed of 300 ° C./min or higher. After cooling at 700 ° C. or lower, cold rolling and continuous annealing, the average size of at least one of (Mn, Cu) S precipitate and AlN precipitate is 0.2 μm. The manufacturing method of the high yield ratio cold-rolled steel plate excellent in the formability which becomes the following. The content of C, Ti, Nb, N, S is the following relationship:
Satisfy ^{0.8 ≦ (Ti ★ /48+Nb/93)/(C/12)≦5.0,Ti ★ =} Ti-0.8 * ((48/14) * N + (48/32) * S) The manufacturing method of the high yield ratio cold-rolled steel plate excellent in formability of Claim 19 or Claim 21 characterized by the above-mentioned.   The method for producing a high yield ratio cold-rolled steel sheet having excellent formability according to claim 22, wherein the C content is 0.005% or less. Wherein C is 5-30 [where Cs (solute carbon) which is determined by the Ti, Cs = (C-Nb * 12/93-Ti ★ * 12/48) * 10000, Ti ★ = Ti-0.8 * ((48/14) * N + (48/32) * S) However, when Ti ^* <0, Ti ^* = 0 is satisfied. A method for producing a high yield ratio cold rolled steel sheet with excellent formability.   The method for producing a high yield ratio cold-rolled steel sheet having excellent formability according to claim 24, wherein the C content is 0.001 to 0.01%.   The method for producing a high yield ratio cold-rolled steel sheet having excellent formability according to any one of claims 19 to 21, wherein a yield ratio (yield strength / tensile strength) is 0.58 or more. . The method for producing a high yield ratio cold-rolled steel sheet having excellent formability according to any one of claims 19 to 21, wherein the number of precipitates is 1 * 10 ⁶ pieces / mm ² or more. .   The method for producing a high yield ratio cold-rolled steel sheet having excellent formability according to claim 19 or 21, wherein the P content is 0.015% or less.   The method for producing a high yield ratio cold-rolled steel sheet having excellent formability according to claim 19 or 21, wherein the P content is 0.03-0.2%.   Furthermore, 1 type or 2 types of Si: 0.1-0.8% and Cr: 0.2-1.2% are contained, It is excellent in the moldability of Claim 19 or Claim 21 characterized by the above-mentioned. A high yield ratio cold rolled steel sheet manufacturing method.   Furthermore, Mo is contained in 0.01-0.2%, The manufacturing method of the high yield ratio cold-rolled steel plate excellent in the formability of Claim 19 or Claim 21 characterized by the above-mentioned.   Furthermore, Mo is contained in 0.01-0.2%, The manufacturing method of the high yield ratio cold-rolled steel plate excellent in the formability of Claim 30 characterized by the above-mentioned.   The method for producing a high yield ratio cold-rolled steel sheet with excellent formability according to claim 21, wherein the sum of Mn and Cu is 0.08-0.4%.   The method for producing a high yield ratio cold-rolled steel sheet having excellent formability according to claim 21, wherein the Mn content is 0.01-0.12%. The method for producing a high yield ratio cold-rolled steel sheet having excellent formability according to claim 21, wherein (Mn / 55 + Cu / 63.5) / (S ^* / 32) satisfies 1-9. Said (Al / 27) / (N ^* / 14) satisfy | fills 1-6, The manufacturing method of the high yield ratio cold-rolled steel plate excellent in the formability of Claim 20 or Claim 21 characterized by the above-mentioned.